Public transport journey planning systems have been developed which provide step by step directions from a start location to a target destination via one or more forms of public transportation (hereinafter referred to as “transit”) taking into account a selected start time or arrival time for the journey. The directions generated by the system may include a sequence of scheduled public transit departures which the user needs to make, and may also specify one or more transfers (e.g: by walking) from the location that one vehicle arrives to the location that another vehicle departs.
Transit data regarding transit schedules of transit vehicles (e.g: trains, trams, buses, ferries etc) may be provided to journey planning systems by one or more transit providers. Transfers and transfer times may be specified in the received data, or may be determined separately by a walking route planner which calculates a walking time between selected locations.
Journey planning systems which provide transit directions may generate a “transit graph” to represent transit data. For a description of transit graph generation, see US patent application publication no.: US2011/0112759, filed 1 Apr. 2010 with title “Transit Routing System for Public Transportation Trip Planning”, which is hereby incorporated herein by reference.
A transit graph may include onboard and station nodes, each onboard/station node belonging to a single station of the transit graph. A station node represents a departing vehicle at a station that a user can board (i.e. enter). An onboard node represents a public transit vehicle that a user is currently boarded, at a given station.
Nodes in the transit graph may be connected to one another via directed arcs, for example:                boarding arcs: These connect a station node to an onboard node;        waiting arcs: These connect a station node to the next (in terms of time) station node at the same station;        transit arcs: These connect an onboard node to an onboard node, and represent travelling between two consecutive stops in a trip;        transfer arcs: These connect an onboard node to a station node. The station node may be in the same station, or could for example be in a nearby station. Transfer arcs represent a user getting off a vehicle represented by the onboard node, and transferring (e.g: walking) to the departing vehicle represented by the station node.        
A transit graph can be interrogated to determine optimal public transit routes in response to a user query, or in some systems, during a pre-processing stage before a query is received. Reference is directed to US 2011/0112759 for a description of an approach which uses pre-processed transit information prior to query time in order to determine optimal public transit routes for journeys.